The Last Word

Question: I've always understood that hot air rises and cold air sinks. But on a recent visit to Death Valley in the US, I learned that it is the hottest and driest area in North America despite being the lowest point in the western hemisphere. Why is this?

Answer: The apparent paradox of warm air lying beneath cold air is not confined to Death Valley. All over the globe, air temperatures tend to decline with height, up to the bottom of the stratospherewitness the presence of permanent snow on high mountaintops. This is partly because most of the radiation from the Sun passes through the air without warming it. It heats instead the land and sea surfaces that lie below, which in turn warm only the air that is immediately above them.

So why does the hot air in Death Valley not push through the cooler air above and continue ...

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